Barium titanate-based (BaTiO3-based) semiconductor ceramic compositions for use in PTC thermistors are known to exhibit an abrupt increase in resistance when being heated to certain temperatures (Curie temperatures). In order that the barium titanate-based semiconductor ceramic compositions can be used at higher temperatures for applications such as thermistors for heaters and increasing the range of applications, increasing the Curie temperatures is required.
A conventional technique for obtaining an increased Curie temperature is to use a (Ba, Pb)TiO3-based semiconductor ceramic composition prepared by partially replacing Ba in a barium titanate-based semiconductor ceramic composition with Pb, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 56-169301 (Patent Document 1).
However, in the case of adding Pb to the barium titanate-based semiconductor ceramic composition to increase the Curie temperature, the resulting temperature coefficient of resistance thereof is small and the composition is voltage-dependent, as disclosed in Patent Document 1. Therefore, barium titanate-based semiconductor ceramic compositions which contain no Pb and which have high Curie temperature are needed.
Among barium titanate-based semiconductor ceramic compositions having high Curie temperature for use in positive temperature coefficient thermistors, a (Ba, Na, Bi)TiO3-based semiconductor ceramic composition prepared by partially replacing Ba with Na and Bi is known, as disclosed in, for example, Patent Document 1 or Japanese Unexamined Patent Application Publication No. 2005-255493 (Patent Document 2).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 56-169301
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-255493